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TELNETD(8)                BSD System Manager's Manual               TELNETD(8)

     telnetd -- DARPA TELNET protocol server

     /usr/libexec/telnetd [-46BUhlkn] [-D debugmode] [-S tos] [-X authtype]
                          [-a authmode] [-edebug] [-p loginprog] [-u len]
                          [-debug [port]]

     The telnetd command is a server which supports the DARPA standard TELNET
     virtual terminal protocol.  Telnetd is normally invoked by the internet
     server (see inetd(8)) for requests to connect to the TELNET port as indi-
     cated by the /etc/services file (see services(5)).  The -debug option may
     be used to start up telnetd manually, instead of through inetd(8).  If
     started up this way, port may be specified to run telnetd on an alternate
     TCP port number.

     The telnetd command accepts the following options:

     -4      Forces telnetd to use IPv4 addresses only.

     -6      Forces telnetd to use IPv6 addresses only.

     -a authmode
             This option may be used for specifying what mode should be used
             for authentication.  Note that this option is only useful if
             telnetd has been compiled with support for the AUTHENTICATION
             option.  There are several valid values for authmode:

             debug  Turn on authentication debugging code.

             user   Only allow connections when the remote user can provide
                    valid authentication information to identify the remote
                    user, and is allowed access to the specified account with-
                    out providing a password.

             valid  Only allow connections when the remote user can provide
                    valid authentication information to identify the remote
                    user.  The login(1) command will provide any additional
                    user verification needed if the remote user is not allowed
                    automatic access to the specified account.

             other  Only allow connections that supply some authentication
                    information.  This option is currently not supported by
                    any of the existing authentication mechanisms, and is thus
                    the same as specifying -a valid.

             none   This is the default state.  Authentication information is
                    not required.  If no or insufficient authentication infor-
                    mation is provided, then the login(1) program will provide
                    the necessary user verification.

             off    Disable the authentication code.  All user verification
                    will happen through the login(1) program.

     -B      Specify bftp server mode.  In this mode, telnetd causes login to
             start a bftp(1) session rather than the user's normal shell.  In
             bftp daemon mode normal logins are not supported, and it must be
             used on a port other than the normal TELNET port.

     -D debugmode
             This option may be used for debugging purposes.  This allows
             telnetd to print out debugging information to the connection,
             allowing the user to see what telnetd is doing.  There are sev-
             eral possible values for debugmode:

             options   Print information about the negotiation of TELNET

             report    Print the options information, plus some additional
                       information about what processing is going on.

             netdata   Display the data stream received by telnetd.

             ptydata   Display data written to the pty.

             exercise  Has not been implemented yet.

     -debug  Enable debugging on each socket created by telnetd (see SO_DEBUG
             in socket(2)).

             If telnetd has been compiled with support for data encryption,
             then the -edebug option may be used to enable encryption debug-
             ging code.

     -h      Disable the printing of host-specific information before login
             has been completed.

     -k      This option is only useful if telnetd has been compiled with both
             linemode and kludge linemode support.  If the -k option is speci-
             fied, then if the remote client does not support the LINEMODE
             option, then telnetd will operate in character at a time mode.
             It will still support kludge linemode, but will only go into
             kludge linemode if the remote client requests it.  (This is done
             by the client sending DONT SUPPRESS-GO-AHEAD and DONT ECHO.)  The
             -k option is most useful when there are remote clients that do
             not support kludge linemode, but pass the heuristic (if they
             respond with WILL TIMING-MARK in response to a DO TIMING-MARK)
             for kludge linemode support.

     -l      Specify line mode. Try to force clients to use line- at-a-time
             mode.  If the LINEMODE option is not supported, it will go into
             kludge linemode.

     -n      Disable TCP keep-alives.  Normally telnetd enables the TCP keep-
             alive mechanism to probe connections that have been idle for some
             period of time to determine if the client is still there, so that
             idle connections from machines that have crashed or can no longer
             be reached may be cleaned up.

     -p loginprog
             Specify an alternate login(1) command to run to complete the
             login.  The alternate command must understand the same command
             arguments as the standard login.

     -S tos  Sets the IP type-of-service (TOS) option for the telnet connec-
             tion to the value tos, which can be a numeric TOS value or, on
             systems that support it, a symbolic TOS name found in the
             /etc/iptos file.

     -u len  This option is used to specify the size of the field in the utmp
             structure that holds the remote host name.  If the resolved host
             name is longer than len, the dotted decimal value will be used
             instead.  This allows hosts with very long host names that over-
             flow this field to still be uniquely identified.  Specifying -u0
             indicates that only dotted decimal addresses should be put into
             the utmp file.

     -U      This option causes telnetd to refuse connections from addresses
             that cannot be mapped back into a symbolic name via the
             gethostbyaddr(3) routine.

     -X authtype
             This option is only valid if telnetd has been built with support
             for the authentication option.  It disables the use of authtype
             authentication, and can be used to temporarily disable a specific
             authentication type without having to recompile telnetd.

     Telnetd operates by allocating a pseudo-terminal device (see pty(4)) for
     a client, then creating a login process which has the slave side of the
     pseudo-terminal as stdin, stdout and stderr.  Telnetd manipulates the
     master side of the pseudo-terminal, implementing the TELNET protocol and
     passing characters between the remote client and the login process.

     When a TELNET session is started up, telnetd sends TELNET options to the
     client side indicating a willingness to do the following TELNET options,
     which are described in more detail below:

           WILL ENCRYPT
           DO TSPEED
           DO XDISPLOC
           DO NEW-ENVIRON
           DO ENVIRON
           DO ECHO
           DO LINEMODE
           DO NAWS
           WILL STATUS
           DO LFLOW
           DO TIMING-MARK

     The pseudo-terminal allocated to the client is configured to operate in
     ``cooked'' mode, and with XTABS and CRMOD enabled (see tty(4)).

     Telnetd has support for enabling locally the following TELNET options:

     WILL ECHO          When the LINEMODE option is enabled, a WILL ECHO or
                        WONT ECHO will be sent to the client to indicate the
                        current state of terminal echoing.  When terminal echo
                        is not desired, a WILL ECHO is sent to indicate that
                        telnetd will take care of echoing any data that needs
                        to be echoed to the terminal, and then nothing is
                        echoed.  When terminal echo is desired, a WONT ECHO is
                        sent to indicate that telnetd will not be doing any
                        terminal echoing, so the client should do any terminal
                        echoing that is needed.

     WILL BINARY        Indicate that the client is willing to send a 8 bits
                        of data, rather than the normal 7 bits of the Network
                        Virtual Terminal.

     WILL SGA           Indicate that it will not be sending IAC GA, go ahead,

     WILL STATUS        Indicate a willingness to send the client, upon
                        request, of the current status of all TELNET options.

     WILL TIMING-MARK   Whenever a DO TIMING-MARK command is received, it is
                        always responded to with a WILL TIMING-MARK.

     WILL LOGOUT        When a DO LOGOUT is received, a WILL LOGOUT is sent in
                        response, and the TELNET session is shut down.

     WILL ENCRYPT       Only sent if telnetd is compiled with support for data
                        encryption, and indicates a willingness to decrypt the
                        data stream.

     Telnetd has support for enabling remotely the following TELNET options:

     DO BINARY          Sent to indicate that telnetd is willing to receive an
                        8 bit data stream.

     DO LFLOW           Requests that the client handle flow control charac-
                        ters remotely.

     DO ECHO            This is not really supported, but is sent to identify
                        a 4.2BSD telnet(1) client, which will improperly
                        respond with WILL ECHO.  If a WILL ECHO is received, a
                        DONT ECHO will be sent in response.

     DO TERMINAL-TYPE   Indicate a desire to be able to request the name of
                        the type of terminal that is attached to the client
                        side of the connection.

     DO SGA             Indicate that it does not need to receive IAC GA, the
                        go ahead command.

     DO NAWS            Requests that the client inform the server when the
                        window (display) size changes.

     DO TERMINAL-SPEED  Indicate a desire to be able to request information
                        about the speed of the serial line to which the client
                        is attached.

     DO XDISPLOC        Indicate a desire to be able to request the name of
                        the X Window System display that is associated with
                        the telnet client.

     DO NEW-ENVIRON     Indicate a desire to be able to request environment
                        variable information, as described in RFC 1572.

     DO ENVIRON         Indicate a desire to be able to request environment
                        variable information, as described in RFC 1408.

     DO LINEMODE        Only sent if telnetd is compiled with support for
                        linemode, and requests that the client do line by line

     DO TIMING-MARK     Only sent if telnetd is compiled with support for both
                        linemode and kludge linemode, and the client responded
                        with WONT LINEMODE.  If the client responds with WILL
                        TM, the it is assumed that the client supports kludge
                        linemode.  Note that the [-k] option can be used to
                        disable this.

     DO AUTHENTICATION  Only sent if telnetd is compiled with support for
                        authentication, and indicates a willingness to receive
                        authentication information for automatic login.

     DO ENCRYPT         Only sent if telnetd is compiled with support for data
                        encryption, and indicates a willingness to decrypt the
                        data stream.

     By default telnetd will read the he, hn, and im capabilities from
     /etc/gettytab and use that information (if present) to determine what to
     display before the login: prompt. You can also use a System V style
     /etc/issue file by using the if capability, which will override im.  The
     information specified in either im or if will be displayed to both con-
     sole and remote logins.

     /etc/iptos     (if supported)
     /usr/ucb/bftp  (if supported)

     bftp(1), login(1), gettytab(5), telnet(1) (if supported)

     RFC-1073  Telnet Window Size Option
     RFC-1079  Telnet Terminal Speed Option
     RFC-1091  Telnet Terminal-Type Option
     RFC-1096  Telnet X Display Location Option
     RFC-1123  Requirements for Internet Hosts -- Application and Support
     RFC-1184  Telnet Linemode Option
     RFC-1372  Telnet Remote Flow Control Option
     RFC-1416  Telnet Authentication Option
     RFC-1411  Telnet Authentication: Kerberos Version 4
     RFC-1412  Telnet Authentication: SPX
     RFC-1571  Telnet Environment Option Interoperability Issues
     RFC-1572  Telnet Environment Option

     Some TELNET commands are only partially implemented.

     Because of bugs in the original 4.2BSD telnet(1), telnetd performs some
     dubious protocol exchanges to try to discover if the remote client is, in
     fact, a 4.2BSD telnet(1).

     Binary mode has no common interpretation except between similar operating
     systems (Unix in this case).

     The terminal type name received from the remote client is converted to
     lower case.

     Telnetd never sends TELNET IAC GA (go ahead) commands.

     IPv6 support was added by WIDE/KAME project.

BSD                             January 9, 2005                            BSD